ANTIBODIES TO CAPSULAR POLYSACCHARIDES ARE NOT PROTECTIVE AGAINST EXPERIMENTAL STAPHYLOCOCCUS-AUREUS ENDOCARDITIS

The protective efficacy of antibodies to the Staphylococcus aureus cap sular polysaccharide was examined in a rat model of catheter-induced e ndocarditis. Capsular antibodies were induced either by active immuniz ation with killed S. aureus or by passive immunization vrith hyperimmu ne rabbit antiserum to S. aureus. Control rats were injected with phos phate-buffered saline or passively immunized with normal rabbit serum or rabbit antiserum to a nonencapsulated strain. Animals with indwelli ng catheters were challenged intravenously with 5 x 10(4) to 4 x 10(6) CFU of the homologous S. aureus strain (capsular serotype 5 strain Re ynolds or serotype 1 strain SA1 mucoid). Both immunized and control ra ts developed S. aureus endocarditis. The numbers of S. aureus cells re covered from the blood and aortic valve vegetations of immunized rats were similar to those of control rats, indicating that capsule-specifi c antibodies were not protective. To determine whether the presence of an indwelling catheter interfered with antibody-mediated protection a gainst S. aureus endocarditis, catheters were removed 2 h after insert ion in additional groups of rats. An inoculum of 10(8) CFU of strain R eynolds was needed to provoke endocarditis in rats catheterized for 2 h, compared with 5 x 10(4) CFU for rats with indwelling catheters. Pas sively transferred capsular antibodies were not protective since both immunized and nonimmunized animals developed endocarditis, and quantit ative cultures of blood and valvular vegetations revealed no differenc es between immunized and control animals. The findings of this study i ndicate that antibodies to the capsular polysaccharide are not protect ive in the rat model of experimental S. aureus endocarditis.